Iron-sulfur clusters are present in more than 120 different types of enzymes or proteins and constitute one of the most ancient, ubiquitous and structurally diverse classes of biological prosthetic groups. Although their primary role lies in mediating biological electron transport, iron-sulfur centers are known to constitute the active sites of numerous enzymes and to have important structural and regulatory roles. However, the functional diversity of biological iron-sulfur clusters has yet to be fully defined, and the mechanism of cluster biosynthesis, which is central to cellular iron homeostasis and the regulatory roles of iron-sulfur clusters, is still poorly understood. The long-term goal of this project is a molecular-level understanding of cluster biosynthesis and of the newly emerging roles of biological iron-sulfur clusters in disulfide reduction, initiating radical reactions in S-adenosylmethionine-dependent enzymes, and providing the sulfur for biosynthesis of biotin and lipoic acid. Ultimately this will lead to enhanced understanding of iron homeostasis and human diseases related to iron overload and defects or inhibition of respiratory chain enzymes. The approach involves using molecular biology techniques to effect large scale expression and/or site-specific changes in the target enzymes and proteins, biochemical and enzymatic assays, and the application of biophysical spectroscopic techniques (electron paramagnetic resonance, absorption, magnetic circular dichroism, resonance, absorption, magnetic circular dichroism, resonance Raman, Mossbauer and mass spectrometry) that can probe the nature and detailed properties of iron or iron-sulfur centers during catalytic cycling or cluster biosynthesis. The specific systems to be investigated include the proteins involved with nitrogen-fixation-specific and general iron-sulfur cluster biosynthesis in Azotobacter vinelandii, biotin synthase from Escherichia coli and ferredoxin:thioredoxin reductase from chloroplasts. The objectives are to establish the mechanism of NifU/NifS- and IscU/IscS-mediated iron-sulfur cluster biosynthesis, determine the role of the iron-sulfur cluster in ferredoxin:thioredoxin reductase in mediating reductive cleavage of the active-site disulfide, characterize the cluster transformation that is responsible for providing the sulfur for biotin biosynthesis, determine the mechanism of iron-sulfur cluster-mediated reductive cleavage of S-adenosylmethionine in biotin synthase, and develop an in vitro catalytic system for biotin biosynthesis.